If you can think of a way to trigger your camera, you can probably do it with TriggerTrap

TriggerTrap is another app which works with an accessory cable to remote trigger you DSLR. Compared to other trigger app/cable combos, TriggerTrap distinguishes itself by also triggering the iPhone’s own camera, should you wish, and by its crazy range of triggering modes.

Using the iPhone's headphone jack, you can control your camera any which way you like

After years of tweaking and improvement, ioShutter is finally here. ioShutter is a simple cable that connects your iPhone to your camera and allows you to control it using an app. Remote shooting, time-lapse sequences and even photos triggered by sound can all be programmed in easily using the free companion app. And best of all, no fancy dock connectors are required: ioShutter connects through the headphone jack.